Plastic Phenotypic Response to Light of 16 Congeneric Shrubs from a Panamanian Rainforest

Abstract

Abstract. The comparative phenotypic plasticity of 16 species of tropical rainforest shrubs (genus Psychotria, Rubiaceae) was investigated by growing plants in three light environments on Barro Colorado Island (BCI, Panama). The three light environments gave daily photon flux densities (PPFD) similar to the natural light gradient from shaded forest understory to small and large canopy gaps. Six of the species are principally found in gaps or forest edge environments, whereas the other ten species are principally found in shaded understories. Interactions between light treatment and species resulted in unpredictable mean phenotypic expression across light treatments. Shoot relative growth rates (RGR) were similar for understory and gap species in the low light treatment. Gap species had significantly greater shoot RGR in the intermediate light treatment than in the high light treatment. Mean plasticity was significantly lower for morphological variables when compared to physiological variables, while variation in plasticity was significantly greater for structural variables. Significant differences between gap and understory species were found in the plasticity of six out of the seven variables. The mean phenotypic plasticity of the seven variables was significantly greater for gap than for understory species. The high plasticity of gap species was consistent with the hypothesis that specialization in a more favorable environment increases plasticity. The species exhibited a wide range of leaf longevities, from four to 29 months, with gap species having, on average, shorter leaf lifespan than understory species. Mean phenotypic plasticity decreased with increasing leaf longevity. Selection for greater plasticity may be stronger in the gap species because gaps exhibit a relatively predictable decrease in PPFD for which plasticity could be adaptive. While we have found a significant correlation between phenotypic plasticity and habitat affiliation, phylogeny (subgenus ascription) was not correlated with plasticity or with plant performance in any given PPFD treatment, reinforcing the hypothesis that phenotypic plasticity has evolved through natural selection in this diverse genus.

Leaf display and photosynthesis of tree seedlings in a cool-temperate deciduous broadleaf forest understorey

“…Leaf-level plasticity to light availability has been argued to be important for the survival and distribution of species along the successional gradients of forests (Bazzaz and Pickett 1980;Reich et al 1992Reich et al , 1999Abrams and Mostoller 1995). However, generally a whole plant growth strategy incorporating biomass allocation patterns, crown architecture and leaf photosynthesis and their interactive effect on carbon gain are important to understand plant growth and survival (Kitajima 1994;Valladares et al 2000). Studies of plant responses in a wider range of resources and stresses, or in habitats having seasonal changes of resources would deepen our understanding of the growth of individual plants, population dynamics, species distributions and mechanisms of succession.…”

“…Effective acquisition and efficient use of light can be critical especially for plants inhabiting the forest understorey where it is spatially and temporally variable and also strongly limiting to plant growth. When plants are grown under understorey light conditions there is indeed a strong convergence of morphological and physiological characteristics of gapand understorey-dependent species onto a shade plant syndrome (e.g., Pearcy and Sims 1994;Ackerly 1996;Chazdon et al 1996;Kitajima 1996;Valladares et al 2000). Differences remain between these contrasting groups or along the successional gradients (StraussDebendetti and Bazzaz 1996) but understanding their role in species habitat preference requires an integrative approach incorporating both morphology and physiology.…”

The combined impacts of deep shade and drought on the growth and biomass allocation of shade-tolerant woody seedlings

“…The study species' SLAs and RMFs in 3.5% daylight were respectively 130-164% and 73-86% of those in 35% daylight, which reflect a moderate degree of plasticity if compared with those of 19 species of temperate woody deciduous seedlings raised a single season in 2-3% daylight and 17-65% daylight (mean values 155% and 82% for the increase of SLA and the reduction of RMF respectively; references for SLA and RMF values provided below). Such moderate morphological plasticity is typical for shade-tolerators (Grime 1979;Grubb 1998;Valladares et al 2000). It is possible that the lower vapor-pressure deficit in deeper shade compensates to a degree for the greater evaporative load resulting from a higher leaf area ratio.…”

Nutritional differences and leaf acclimation of climbing plants and the associated vegetation in different types of an Andean montane rainforest

“…Along forest edges and in tree-fall gaps climbers build large curtains and reach their highest abundance (Pen˜alosa 1983;Uhl et al 1988), suppressing trees. Growing often in gaps, they exhibit typical features like high growth rates and rapid leaf turnover (Valladares et al 2000), probably accompanied by several other leaf traits. Negative impact on forest regeneration is strong as climbers reduce tree survival and re-growth by mechanical damages and shading (Putz 1984;Schnitzer and Bongers 2002).…”